The present invention provides a hard, scratch-resistant, easily washed protective coating for substrates.
Glass panes employed in windows and doors, for example, or as mirrors, have outer surfaces that require periodic washing, as with a detergent and water. The surface of sheet glass itself commonly exhibits a high contact angle with the water. That is, water, when applied to the surface of the glass, tends to form discrete droplets or beads. Evaporation of the water beads leaves the entrained or dissolved dirt in the droplets on the glass. In order to be readily cleaned, the surfaces of glass panes should be able to xe2x80x9csheet awayxe2x80x9d water droplets; that is, the droplets themselves should not bead up but rather should spread out, coalesce, and drain from the glass surface. To achieve this result, the glass surface should form a contact angle with water droplets of not greater than about 25xc2x0.
Glass sheets of the type used in windows and doors often bear on one or more surfaces in optical film stacks. Such film stacks are well known in the field, and certain of these coatings have been referred to as xe2x80x9csolar mirrorsxe2x80x9d, xe2x80x9clow emissivity coatingsxe2x80x9d, etc. They commonly involve the use of a infrared reflective metallic film such as silver together with one or more dielectric films such as zinc oxide. To protect optical film stacks of this type, an outer, protective coating of a nitride such as silicon nitride may be employed. The surface of a silicon nitride generally provides an acceptably low contact angle with water, but silicon nitride itself is not as hard and scratch resistant as is needed. Silicon carbide, on the other hand, is quite hard and scratch resistant, but it, unfortunately, is quite hydrophobic; surfaces of silicon carbide provide high contact angles with water, and accordingly, silicon carbide surfaces become easily spotted and are not easily washed.
It would be desirable to provide an outer, protective coating for a glass substrate that would combine the characteristics of protecting from chemical attack any underlying film stack, provide having a hard and scratch-resistant surface, and yet exhibit good washability.
It has now been found that silicon carbide and silicon nitride can be combined in a single magnetron sputtered film that is hard and scratch-resistant on the one hand, but that exhibits a low contact angle with water on the other. The protective silicon carbide/silicon nitride coatings of the invention are amorphous, and vary in thickness from about 30 xc3x85 to about 500 xc3x85. Water contact angles of the surfaces of these protective coatings is not greater than about 25xc2x0, and preferably not greater than about 15xc2x0. The protective coatings themselves are manufactured utilizing a magnetron sputtering procedure in which silicon from a silicon target is sputtered in an atmosphere containing both nitrogen and a hydrocarbon gas, preferably methane. The resulting film appears to be a mixed nitride and carbide of silicon, the stoichiometry of the system being susceptible to control through control of the reactive atmosphere in the sputtering chamber. The hydrocarbon gas itself provides hydrogen atoms which, it is believed, is included in the protective coating to fill empty lattice positions and force the correct bonding configuration to result in a hard coating.
The protective silicon nitride/carbide film of the invention preferably is sputtered onto a glass sheet in a magnetron sputtering line comprising a series of sequential magnetron chambers. For reasons of economy, it is greatly desired to perform all of the sputter coating operations on glass sheets in a single pass of the glass sheets through the magnetron coating line, rather than going through the labor-intensive task of collecting coated sheets and physically re-running them through a magnetron sputtering line to apply additional layers. If the protective coating of the invention is to be placed upon an optical film stack carried on one surface of the glass sheet, one or more magnetron sputtering chambers may be employed downstream from the series of sequential magnetron sputtering chambers used to deposit film elements of the optical film stack, the latter chambers being employed to deposit the protective silicon nitride/carbide coating of the invention. If the protective coating is to be applied to the other side of the glass sheet, this too may be accomplished in a single pass of the glass sheet through a series of magnetron coating chambers by sputtering the reverse side of the glass sheet with the silicon nitride/carbide coating of the invention.
Thus, in one embodiment, the invention provides a method of producing a hard, scratch-resistant, easily washed coating on a substrate such as glass. The method comprises introducing the substrate into a magnetron sputtering chamber and sputtering silicon from a silicon target while maintaining in the chamber a reactive atmosphere that includes nitrogen and a hydrocarbon gas, the method producing on the substrate surface an amorphous protective film containing a carbide and a nitride of silicon. The resulting film has a thickness in the range of about 30 xc3x85 to about 500 xc3x85, and has a surface exhibiting a contact angle with water of below about 25xc2x0 and preferably below 15xc2x0, with water contact angles of 8xc2x0 and below being achievable.
Preferably, the molar concentration of hydrocarbon gas in the reactive atmosphere of hydrocarbon gas and nitrogen within the sputtering chamber ranges from about 5% to about 35%. Further, the atomic percentage of carbon to carbon plus nitrogen in the total silicon alloy preferably ranges from about 5% to about 35%.
In another embodiment, the method as defined above is performed in a single magnetron sputtering line and involves the steps of passing the substrate sequentially through a series of magnetron sputtering chambers including one series of chambers for sputtering on one surface of the glass a series of films forming an optical film stack, the sputtering line including at least one chamber in which is sputtered onto the film stack the hard, scratch-resistant coating of the invention. In another embodiment, a single magnetron sputtering line is employed, the line including one series of chambers for sputtering on one surface of the glass a series of films forming an optical film stack, and at least one chamber for sputtering on the reverse side of the substrate the hard, scratch-resistant silicon nitride/carbide coating of the invention.
In another embodiment, the invention relates to a film stack carried on a substrate such as glass. The film stack comprises, from the substrate outwardly, an optical film stack and a hard, scratch-resistant outer protective layer, the latter comprising an amorphous film containing a carbide of silicon and a nitride of silicon. The protective coating has a thickness in the range of about 30 xc3x85 to about 500 xc3x85, and has an outer surface exhibiting a contact angle with water not greater than about 25xc2x0. The structure of the protective coating is that which is produced by sputtering silicon in a reactive atmosphere containing both nitrogen and a hydrocarbon gas, and results in the inclusion of hydrogen in the protective coating.